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(CNN) -- When Discovery thunders off the launch pad at Kennedy Space Center in Florida, one question will be on everyone's mind: Is it safe?

NASA has asked itself that question obsessively over the past two years since the loss of the space shuttle Columbia. Now, the agency claims it can finally answer it.

"This is the safest vehicle we've ever flown," says Bill Parsons, NASA's space shuttle program manager. He credits NASA engineers who conducted an unprecedented testing program examining the shuttle in greater detail than was thought possible when the first orbiter launched in 1983.

"We know a lot more about this vehicle that we did two years ago," Parsons said.

Air cannons have bombarded spare shuttle parts with pieces of foam. Fighter jets have launched chunks of debris to study their aerodynamics. The Columbia Project, one of the world's fastest supercomputers, spent the last year and a half simulating the trajectory of 1 billion possible objects striking the shuttle during launch, according to NASA.

Despite the effort, doubts about the shuttle's safety persist. In fact, the space shuttle remains an "experimental" spacecraft. The vehicle has never delivered on the promise of routine access to space. According to an analysis by NASA engineers made during the investigation into the 1986 Challenger explosion, the shuttle has a failure rate approaching 1 in 100.

Those odds may not have changed much for Discovery.

"As of this day, it's my understanding we are still going to fly with some risk," said Wayne Hale, deputy shuttle manager. "To characterize otherwise would be inappropriate ... (The risk) is very, very low, but I don't think we've driven it to zero."

The Columbia Accident Investigation Board, or CAIB, the independent panel charged with investigating the accident, said eliminating the physical cause of the Columbia accident -- foam falling off the external fuel tank during launch and striking the shuttle -- is only the beginning.

"NASA's organizational culture had as much to do with this accident as foam did," the report states.

Beyond technical fixes, the board said NASA must uproot an entrenched complacency about the shuttle program, the legacy of NASA's transition from a Cold War race with the Soviets in the 1960s to an underfunded bureaucracy during the 1980s.

CAIB conducted a seven-month investigation involving 120 researchers and 400 NASA and contractor employees. NASA says all 15 of CAIB's recommendations -- including better imaging, emergency repair options for the astronauts and addressing flaws with the external fuel tank -- are being addressed before Discovery is launched.

On June 27, an independent panel concluded that NASA failed to meet three of the 15 safety recommendations but also acknowledged that their high standard was probably not achievable "within the technology, funding and schedule available to the Space Shuttle Program," the report stated.

One of CAIB's primary recommendations -- upholding safety standards under pressure from budget and scheduling demands.

The report said NASA's assurances of safety "cannot be trusted" because of previous assurances before the losses of Challenger and Columbia.

Safety first

In response, NASA has embarked an overhaul of its safety culture.

NASA hired California-based Behavioral Science Technology to change the culture among the 19,000 employees and contractors that work on the shuttle program. According to the firm, NASA has improved on all 11 criteria in a recent safety survey, posting some of the best results in the company's history.

"Culture change takes time, and NASA's culture is definitely improving," said Dr. Thomas Krause, chairman of Behavioral Science Technology. "Based on its success, the change method is now being taken to each center agency-wide."

NASA's top management has been reshuffled, including an agency-wide review of its safety culture down to adding C-shaped conference tables that encourage better communication. The agency established an independent Engineering and Safety Center for objective analysis and tracks an "anomaly list" of abnormalities with the shuttle.

The results will be scrutinized when Discovery takes off from Cape Canaveral.

Members of the launch control center monitor testing on the external tank.

"We are absolutely operating in a fishbowl," Hale said at the Johnson Space Center in April. "It means that every day you have to tell the truth. There's not an option to say anything but the absolute truth."

Nobel-prize winning physicist Richard Feynman, who sat on the Challenger investigation panel, said in past statements that NASA chronically exaggerated the shuttle's safety "to the point of fantasy." That allowed a lax culture to develop prior to the losses of Challenger and Columbia.

In both cases, CAIB reports mission managers were swayed by a demanding timetable and misplaced confidence in the shuttle. They systematically ignored warning signs that the vehicle was operating outside design limits. In its 2003 report, CAIB raised many of the same points as those raised in the Challenger investigation, saying NASA "(relied) on past success as a substitute for sound engineering practices."

Although the shuttle was not a routine, economical or "operational" vehicle, it was being treated like one, said NASA.

"(We were) making more and more decisions with less and less engineering data to support them," said Paul Hill, flight director for the Discovery mission.

In Columbia's case, NASA officials reviewed footage of liftoff showing a briefcase-size chunk of foam hitting the left wing. They considered alerting the Department of Defense to photograph the shuttle, mistakenly assuming the tiles -- not the wing's edge -- was struck, but failed to follow-up. Worried engineers in NASA's mission control never voiced their concerns, said flight managers.

When Columbia entered Earth's atmosphere, superheated gases entered and melted the aluminum airframe in the left wing. The shuttle began disintegrating over California and finally broke up over western Texas.

The disaster, the Columbia board concluded, was caused by a safety culture that confused survival with future success. CAIB said it had "no confidence" that the shuttle program could be safely operated for more than a few years based on post-accident vigilance.

But NASA insists it will not make the same mistake twice. It says the "can-do" agency is now demanding that its managers, engineers and scientists ask each other what they should do.

"People are having to prove why it's safe to fly rather than why it's not safe to fly," Hale said. "This management team today has said, 'If we can't do it the right way, we shouldn't do it at all.' We're willing to stand up and tell our bosses, the American people, if we can't do it the right way."

Keith Cowling, a former NASA scientist who now runs the independent watchdog publication nasawatch.com, said the agency could be right this time. He said NASA has made progress compared to his days there nearly 10 years ago.

"Just flying the shuttle flights needed to complete the (international space) station, that's something I think NASA can get its arms around," Cowling said. "It's a short enough time span that I suspect the lessons won't go unlearned."

Hill, who will head mission control when Discovery launches later this week, says it will be a matter of vigilance and attention to detail.

"Every single launch is a damned dangerous proposition," he said. "I tell (the flight controllers) ... 'don't let me screw this up. If you guys see me leading us down the wrong path, speak up 'cause we're all going to go down together.'

"This isn't just about not hurting my feelings," he said. "This is about getting the right answer."